Structure and Activity Transition from Oxidized to Metallic Tungsten for Catalytic Hydrogenation: A Density Functional Theory Study C

Oxide-based hydrogenation catalysts have attracted intensive interest, but the relationships between their composition, structure, and reaction mechanism are still ambiguous. Here, we conducted density functional theory (DFT) calculation on ethylene hydrogenation over WO₃, WO₂.₇₂, WO₂, and W catalysts to explore how the structure and catalytic activity change with the composition and explain why neither WO₃ nor metallic W is a good hydrogenation catalyst but WO₃–ₓ is. Calculations on the geometric and electronic structures show a transition from semiconductor to metal-like with more W–W metal bonds appearing from the corresponding oxidized to metallic tungsten. Correspondingly, the H₂ dissociation mechanism changes from heterotypic on the semiconductor WO₃ and WO₂.₇₂ to homotypic on the metal-like WO₂ and metallic W, and the adsorption strength of the dissociation product (2H) enhances from oxidized to metallic W. Calculations on the stepwise hydrogenation indicate that H₂ dissociation is the rate-limiting step (RLS) for WO₃ and WO₂.₇₂, whereas the following hydrogenation step (C₂H₅ + H → C₂H₆) is the RLS for WO₂ and W. By linear fitting the binding energies of H₂, 2H, H + C₂H₅, and C₂H₆ adsorptions, a perfect correlation between (H + C₂H₅) and (2H) adsorptions is observed; thus, the catalytic activity and mechanism can be evaluated using 2H adsorption behavior as the descriptor. On the basis of the calculation results, we predict that the best ethylene hydrogenation activity will show at 2 < x < 2.72 for WOₓ. This work suggests that controlling the composition between the stoichiometric oxide and the metal may be an effective way to develop an active hydrogenation catalyst.